Heat shrinkable, coextruded polyethylene film laminate

ABSTRACT

A heat shrinkable, coextruded polyethylene film laminate having two surface layers, and an intermediate layer interposed therebetween, wherein each of the surface layers comprises a linear low density polyethylene having a density of 0.910-0.930 g/cm 3  and a linear high density polyethylene having a density of 0.925-0.945 g/cm 3  and wherein the intermediate layer comprises a linear low density polyethylene having a density of 0.910-0.930 g/cm 3  and a linear very low density polyethylene having a density of 0.880-0.915 g/cm 3 .

BACKGROUND OF THE INVENTION

[0001] This invention relates to a heat shrinkable, coextrudedpolyethylene film laminate suitable for the heat-shrink packaging of anarticle.

[0002] A heat-sealable, biaxially oriented polyethylene film laminate isknown which includes a core layer of a relatively higher melting pointpolyethylene between two surface layer of a relatively low melting pointpolyethylene. A heat-resisting, biaxially oriented polyethylene filmlaminate is also known which includes a core layer of a relatively lowmelting point polyethylene between two surface layer of a relativelyhigh melting point polyethylene. Because of poor stretchability ofpolyethylene in comparison with polypropylene resin, however, it isdifficult to prepare the above polyethylene film laminates. Inparticular, the temperature range in which the stretching ofpolyethylene films can be carried out smoothly in a satisfactory manneris so narrow that it is difficult to continue stretching of thepolyethylene films in a stable manner for a long time. As a consequence,known oriented polyethylene film laminates have defects that thethickness thereof is not uniform and that stretching cannot be continuedin a stable manner for a long process run.

SUMMARY OF THE INVENTION

[0003] It is, therefore, an object of the present invention to provide abiaxially oriented, coextruded polyethylene film laminate which hasuniform thickness.

[0004] Another object of the present invention is to provide a biaxiallyoriented, coextruded polyethylene film laminate which has goodstretchability in both high and low temperaturess and which can becontinuously stretched in a stable manner even when the stretchingtemperature fluctuates.

[0005] It is another object of the present invention to provide abiaxially oriented, coextruded polyethylene film laminate of theabove-mentioned type which shows good heat resistance and good surfaceslippage (good wrapping or packaging property).

[0006] In accomplishing the foregoing objects, there is provided inaccordance with the present invention a polyethylene laminate filmcomprising two surface layers F1 and F2, and an intermediate layer Minterposed therebetween,

[0007] wherein the surface layer F1 comprises

[0008] a linear low density polyethylene having a density D_(AF1) g/cm³of 0.910-0.930 g/cm³ and

[0009] a linear high density polyethylene having a density D_(CF1) g/cm³of 0.925-0.945 g/cm³ and being present in an amount of W_(CF1) % basedon a total weight of the linear low density polyethylene having thedensity D_(AF1) and the linear high density polyethylene having thedensity D_(CF1),

[0010] wherein the surface layer F2 comprises

[0011] a linear low density polyethylene having a density D_(AF2) g/cm³of 0.910-0.930 g/cm³ and

[0012] a linear high density polyethylene having a density D_(CF2) g/cm³of 0.925-0.945 g/cm³ and being present in an amount of W_(CF2) % basedon a total weight of the linear low density polyethylene having thedensity D_(AF2) and the linear high density polyethylene having thedensity D_(CF2),

[0013] wherein the intermediate layer M comprises

[0014] a linear low density polyethylene having a density D_(AM) g/cm³of 0.910-0.930 g/cm³ and

[0015] a linear very low density polyethylene having a density D_(BM)g/cm³ of 0.880-0.915 g/cm³ and being present in an amount of W_(BM) %based on a total weight of the linear low density polyethylene havingthe density D_(AM) and the linear very low density polyethylene havingthe density D_(BM), and

[0016] wherein the laminate film satisfies the following conditions (a)through (e) at the same time:

[0017] (a) D_(CF1)−D_(AF1)≧0.010,

[0018] (b) D_(CF2)−D_(AF2)≧0.010,

[0019] (c) D_(AM)−D_(BM)>0.010,

[0020] (d)0.01≦{(D_(CF1)−D_(AF1))×L_(F1)×W_(CF1)}+{(D_(CF2)−D_(AF2))×L_(F2)×W_(CF2)}≦0.20,

[0021] (e) 0.05≦(D_(AM)−D_(BM))×L_(M)×W_(BM)}≦1.40,

[0022] where D_(CF1), D_(AF1), D_(CF2), D_(AF2), D_(AM), D_(BM),W_(CF1), W_(CF2) and W_(BM) are as defined above and L_(F1), L_(F2) andL_(M) represent as follows:

[0023] L_(F1)=t_(F1)/(t_(F1)+t_(F2)+t_(M)),

[0024] L_(F2)=t_(F2)/(t_(F1)+t_(F2)+t_(M)),

[0025] L_(M)=t_(M)/(t_(F1)+t_(F2)+t_(M)), where t_(F1), t_(F2) and t_(M)represent the thicknesses of the surface layer F1, surface layer F2 andintermediate layer M, respectively.

[0026] Other objects, features and advantages of the present inventionwill become apparent from the detailed description of the preferredembodiments of the invention to follow.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

[0027] In the polyethylene film laminate according to the presentinvention, a linear low density polyethylene (hereinafter referred to asLLDPE) having a density of 0.910-0.930 g/cm³ is used in each of surfacelayers F1 and F2 and in an intermediate layer M interposed therebetween.It is preferred that LLDPE have a melt index of 0.5-5.9 g/10 min forreasons of good streatchability. LLDPE is preferably a copolymer ofethylene with an α-olefin having 4-8 carbon atoms. Copolymers producedusing a Ziegler Natta catalyst or a single site catalyst may be used forthe purpose of the present invention.

[0028] In addition to LLDPE, each of the surface layers F1 and F2contains linear high density polyethylene (hereinafter referred to asLHDPE) having a density of 0.925-0.945 g/cm³. The intermediate layer Mmust contain linear very low density polyethylene (hereinafter referredto as VLDPE) having a density of 0.880-0.910 g/cm³ in addition to LLDPE.

[0029] It is preferred that LVLDPE have a melt index similar to LLDPEand in the range of 0.5-5.0 g/10 min for reasons of good streatchabilityand of compatibility with LLDPE. LVLDPE is preferably a copolymer ofethylene with an α-olefin having 4-8 carbon atoms. Copolymers producedusing a Ziegler Natta catalyst or a single site catalyst may be used forthe purpose of the present invention.

[0030] It is preferred that LHDPE have a melt index similar to LLDPE andin the range of 0.5-5.0 g/10 min for reasons of good streatchability andof compatibility with LLDPE. LHDPE is preferably a copolymer of ethylenewith an α-olefin having 4-8 carbon atoms. Copolymers produced using aZiegler Natta catalyst or a single site catalyst may be used for thepurpose of the present invention.

[0031] In the present invention, it is not necessary that the samepolyethylene be used in the surface and intermediate layers of the samelaminate. Thus, different LLDPEs can used in respective layers of thesame laminate. Similarly, respective layers of the same laminate cancontain different LVLDPEs and different LHDPES.

[0032] The surface layers F1 and F2 and the intermediate layer M of thelaminate according to the present invention must meet with specificconditions. These conditions will be next described.

[0033] As described above, the surface layers F1 and F2 contain LHDPE inaddition to LLDPE, while the intermediate layer M contains LVLDPE inaddition to LLDPE. In this case, it is important that the followingconditions (a) through (c) are met at the same time:

[0034] (a) D_(CF1)−D_(AF1)≧0.010,

[0035] (b) D_(CF2)−D_(AF2)≧0.010,

[0036] (c) D_(AM)−D_(BM)≧0.010,

[0037] wherein

[0038] D_(CF1): density (g/cm³) of LHDPE of surface layer F1

[0039] D_(AF1): density (g/cm³) of LLDPE of surface layer F1

[0040] D_(CF2): density (g/cm³) of LHDPE of surface layer F2

[0041] D_(AF2): density (g/cm³) of LLDPE of surface layer F2

[0042] D_(BM): density (g/cm³) of LVLDPE of intermediate layer M

[0043] D_(AM): density (g/cm³) of LLDPE of intermediate layer M.

[0044] Namely, in each of the surface layers F1 and F2, the density ofLHDPE must be greater by at least 0.010 g/cm³ than that of LLDPE, inorder to obtain good stretchability and heat resistance. At the sametime, in the intermediate layer M, the density of LLDPE must be greaterby at least 0.010 g/cm³ than that of LVLDPE in order to obtain goodstretchability

[0045] Further, it is important that the following conditions (d) and(e) should be met simultaneously:

[0046] (d) 0.01≦Z_(S1)+Z_(S2)≦0.20,Z_(S1)=(D_(CF1)−D_(AF1))×L_(F1)×W_(CF1)Z_(S2)=(D_(CF2)−D_(AF2))×L_(F2)×W_(CF2)

[0047] (e) 0.05≦Z_(M)≦1.40 Z_(M)=(D_(AM)−D_(BM))×L_(M)×W_(BM)

[0048] where D_(AF1), D_(CF1), D_(AF2), D_(CF2), D_(BM) and D_(AM) areas defined above, W_(CF1), W_(CF2) and W_(BM) represent as follows:

[0049] W_(CF1): amount of LHDPE (% by weight) based on a total weight ofLLDPE and LHDPE of the surface layer F1,

[0050] W_(CF2): amount of LHDPE (% by weight) based on a total weight ofLLDPE and LHDPE of the surface layer F2,

[0051] W_(BM): amount of LVLDPE (% by weight) based on a total weight ofLLDPE and LVLDPE of the intermediate layer M,

[0052] and L_(F1), L_(F2 and L) _(M) represent as follows:

[0053] L_(F1)=t_(F1)/(t_(F1)+t_(F2)+t_(M)),

[0054] L_(F2)=t_(F2)/(t_(F1)+t_(F2)+t_(M)),

[0055] L_(M)=t_(M)/(t_(F1)+t_(F2)+t_(M)),

[0056] where t_(F1), t_(F2) and t_(M) represent the thicknesses of thesurface layer F1, surface layer F2 and intermediate layer M,respectively.

[0057] When (Z_(S1)+Z_(S2)) is smaller than 0.01, the range oftemperature in which the laminate can be stretched in a stable manner ata high temperature is so small that it is difficult to stretch thelaminate at such a high temperature for a long period of time. Further,the laminate becomes poor in heat resistance. When (Z_(S1)+Z_(S2)) isgreater than 0.20, on the other hand, it is possible to stretch thelaminate at a high temperature. However, the strechability becomes nogood and the thickness of the stretched film becomes non-uniform. Verygood stretchability and surface slippage (wrapping property) areobtainable when (Z_(S1)+Z_(S2)) is in the range of 0.04 to 0.12 and thisrange represents the preferred range.

[0058] When Z_(M) is smaller than 0.05, the range of temperature inwhich the laminate can be stretched in a stable manner at a lowtemperature is so small that it is difficult to stretch the laminate atsuch a low temperature. When Z_(M) is greater than 1.4, on the otherhand, it is not possible to stretch the laminate at a low temperature ina stable manner. When a hard package is desired, it is preferred thatZ_(M) be in the range of 0.10 to 0.30. On the other hand, when a softpackage is desired, such as for wrapping sheets or cards, it ispreferred that Z_(M) be in the range of 0.70 to 1.20.

[0059] If desired, one or more layers may be interposed between thesurface layers S1 and S2. For example, a layer of a resin obtained fromrecycled laminates or noncompliant laminates may be incorporated intothe laminate of the present invention as an additional intermediatelayer. In such a case, the additional layer may be sandwiched betweentwo intermediate layers M, with surface layers S1 and S2 being providedon opposite sides to form a five-layered laminate. The laminategenerally has a thickness of 8-50 μm, preferably 10-35 μm. The thicknessof each of the surface layers S1 and S2 is generally 5-33, preferably10-25% of the total thickness.

[0060] Each of the surface layers S1 and S2 and the intermediate layer Mmay contain one or more conventional additives such as an antioxidizingagent, an antistatic agent, a lubricant, an antiblocking agent, acolorant and a filler. A crosslinking agent, a crosslinking accelerator,etc, may also be incorporated into the laminate to crosslink thelaminate before or after stretching. Further, a resin such as a highpressure low density polyethylene having a density of 0.91-0.93 may beused in conjunction with LLDPE, LHDPE and/or LVLDPE in an amount of lessthan 15% by weight based on the laminate.

[0061] The laminate of the present invention may be prepared by anysuitable conventional method such as coextrusion. The extruded sheet isstretched, preferably biaxially, by tentering or inflation method toobtain a heat-shrinkable, coextruded polyethylene film laminate suitablefor the heat-shrink packaging of an article. Packages obtained by usingthe heat-shrinkable laminate of the present invention do not melt orwhiten during their passage through a heat-shrinking tunnel. Further,the heat-shrunken films of the packages are free of non-shrunkenportions and are maintained in a tense state.

[0062] The following examples will further illustrate the presentinvention. Parts and percentages are by weight.

EXAMPLE 1

[0063] A resin composition containing 90 parts of LLDPE having a densityof 0.920 g/cm³ and 10 parts of LHDPE having a density of 0.935 g/cm³ wasused for surface layers S1 and S2. A resin composition containing 70parts of LLDPE having a density of 0.920 g/cm³ and 30 parts of LVLDPEhaving a density of 0.900 g/cm³ was used for an intermediate layer M.Each of the composition was charged in an extruder and coextrudedthrough a circular die to form a tubular, three-layered laminate havinga thickness ratio of S1:M:S2 of 15:70:15. The extrudate was rapidlycooled, stretched by 5 times the original length in each of the lateraland machine directions by inflation and then annealed to obtain abiaxially oriented laminate having a thickness of 19 μm.

[0064] The biaxially oriented laminate was evaluated for stability instretching operation, uniformity in thickness and slippage. The resultsare summarized in Table 1. Evaluation is in accordance with thefollowing ratings.

[0065] Stability:

[0066] Inflation stretching for the production of the biaxially orientedlaminate is continuously performed for 24 hours.

[0067] A: excellent (stretching is performed in stable manner andsatisfactorily from industrial viewpoint)

[0068] B: no good (stretching is performed in unstable manner and notsatisfactorily from industrial viewpoint)

[0069] C: poor (industrial production is impossible)

[0070] Uniformity:

[0071] Uniformity in thickness of stretched laminate is checked by nakedeyes and with a thickness gauge.

[0072] A: excellent (non-uniform stretching or non-uniform thickness isnever observed with naked eyes or thickness gauge)

[0073] B: no good (non-uniformity is not observed with naked eyes but isobserved by measurement with the gauge

[0074] C: non-uniform thickness is observed with naked eyes

[0075] Slippage:

[0076] Video tape cartridges are heat-shrink packaged with the laminate.The outer surfaces of packages are manually rubbed with each other toevaluate slippage.

[0077] A: slip

[0078] B: slightly slip

[0079] C: non-slip

EXAMPLE 2

[0080] Example 1 was repeated in the same manner as described exceptthat a resin composition containing 70 parts of LLDPE having a densityof 0.920 g/cm³ and 30 parts of LHDPE having a density of 0.935 g/cm³ wasused for each of the surface layers S1 and S2. The results aresummarized in Table 1.

EXAMPLE 3

[0081] Example 1 was repeated in the same manner as described exceptthat a resin composition containing 30 parts of LLDPE having a densityof 0.920 g/cm³and 70 parts of LVLDPE having a density of 0.900 g/cm³ wasused for the intermediate layer M.

EXAMPLE 4

[0082] Example 3 was repeated in the same manner as described exceptthat a resin composition containing 60 parts of LLDPE having a densityof 0.920 g/cm³ and 40 parts of LHDPE having a density of 0.935 g/cm³ wasused for each of the surface layers S1 and S2. The results aresummarized in Table 1.

Comparative Example 1

[0083] Example 1 was repeated in the same manner as described exceptthat a resin composition containing 50 parts of LLDPE having a densityof 0.920 g/cm³ and 50 parts of LHDPE having a density of 0.935 g/cm³ wasused for each of the surface layers S1 and S2. The results aresummarized in Table 1.

Comparative Example 2

[0084] Example 1 was repeated in the same manner as described exceptthat a resin composition containing 10 parts of LLDPE having a densityof 0.920 g/cm³ and 90 parts of LVLDPE having a density of 0.900 g/cm³was used for the intermediate layer M and that the thickness ratioS1:M:S2 was changed to 10:80:10. The results are summarized in Table 1.

Comparative Example 3

[0085] Example 1 was repeated in the same manner as described exceptthat a resin composition containing 90 parts of LLDPE having a densityof 0.920 g/cm³ and 10 parts of LVLDPE having a density of 0.910 g/cm³was used for the intermediate layer M and that the thickness ratioS1:M:S2 was changed to 30:40:30. The results are summarized in Table 1.

Comparative Example 4

[0086] Example 1 was repeated in the same manner as described exceptthat a resin composition containing 96 parts of LLDPE having a densityof 0.920 g/cm³ and 4 parts of LHDPE having a density of 0.930 g/cm³ wasused for each of the surface layers S1 and S2 and that the thicknessratio S1:M:S2 was changed to 10:80:10. The results are summarized inTable 1. TABLE 1 Example D_(CF1) − D_(CF2) − D_(AM) − Z_(S1) + Sta-Uniform- Slip- No. D_(AF1) D_(AF2) D_(BM) Z_(S2) Z_(M) bility ity page 10.015 0.015 0.02 0.045 0.42 A A A 2 0.015 0.015 0.02 0.135 0.42 A A A 30.015 0.015 0.02 0.045 0.98 A A A 4 0.015 0.015 0.02 0.18  0.98 A A ACmp. 1 0.015 0.015 0.02 0.225 0.42 C C A Cmp. 2 0.015 0.015 0.02 0.03 1.44 C C B Cmp. 3 0.015 0.015 0.01 0.09  0.04 C C A Cmp. 4 0.01  0.01 0.02 0.008 0.48 B B C

[0087] The invention may be embodied in other specific forms withoutdeparting from the spirit or essential characteristics thereof. Thepresent embodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all the changes which come within the meaning and rangeof equivalency of the claims are therefore intended to be embracedtherein.

What is claimed is:
 1. A heat shrinkable, coextruded polyethylene filmlaminate comprising two surface layers F1 and F2, and an intermediatelayer M interposed therebetween, wherein the surface layer F1 comprisesa linear low density polyethylene having a density D_(AF1) g/cm³ of0.910-0.930 g/cm³ and a linear high density polyethylene having adensity D_(CF1) g/cm³ of 0.925-0.945 g/cm³ and being present in anamount of W_(CF1) % based on a total weight of the linear low densitypolyethylene having the density D_(AF1) and the linear high densitypolyethylene having the density D_(CF1), wherein the surface layer F2comprises a linear low density polyethylene having a density D_(AF2)g/cm³ of 0.910-0.930 g/cm³ and a linear high density polyethylene havinga density D_(CF2) g/cm³ of 0.925-0.945 g/cm³ and being present in anamount of W_(CF2) % based on a total weight of the linear low densitypolyethylene having the density D_(AF2) and the linear high densitypolyethylene having the density D_(CF2), wherein the intermediate layerM comprises a linear low density polyethylene having a density D_(AM)g/cm³ of 0.910-0.930 g/cm³ and a linear very low density polyethylenehaving a density D_(BM) g/cm³ of 0.880-0.915 g/cm³ and being present inan amount of W_(BM) % based on a total weight of the linear low densitypolyethylene having the density D_(AM) and the linear very low densitypolyethylene having the density D_(BM), and wherein the laminate filmsatisfies the following conditions (a) through (e) at the same time: (a)D_(CF1)−D_(AF1)≧0.010, (b) D_(CF2)−D_(AF2)≧0.010, (C)D_(AM)−D_(BM)≧0.010, (d)0.01≦{(D_(CF1)−D_(AF1))×L_(F1)×W_(CF1)}+{(D_(CF2)−D_(AF2))×L_(F2)×W_(CF2)}≦0.20,(e) 0.05≦(D_(AM)−D_(BM))×L_(M)×W_(BM)}≦1.40, where D_(CF1), D_(AF1),D_(CF2), D_(AF2), D_(AM)D_(BM), W_(CF1), W_(CF2) and W_(BM) are asdefined above and L_(F1), L_(F2) and L_(M) represent as follows:L_(F1)=t_(F1)/(t_(F1)+t_(F2)+t_(M)),L_(F2)=t_(F2)/(t_(F1)+t_(F2)+t_(M)), L_(M)=t_(M)/(t_(F1)+t_(F2)+t_(M)),where t_(F1), t_(F2) and t_(M) represent the thicknesses of the surfacelayer F1, surface layer F2 and intermediate layer M, respectively.
 2. Alaminate film as recited in claim 1, and having a total thickness of8-50 μm and wherein the thickness of each of the surface layers is 5-33%of the total thickness.
 3. A laminate film as recited in claim 1, andsatisfying the following conditions (d′) and (e′): (d′)0.04≦{(D_(CF1)−D_(AF1))×L_(F1)×W_(CF1)}+{(D_(CF2)−D_(AF2))×L_(F2)×W_(CF2)}≦0.12,(e′) 10≦(D_(AM)−D_(BM))×L_(M)×W_(BM)}≦30, wherein D_(CF1), D_(AF1),D_(CF2), D_(AF2), D_(AM), D_(BM), W_(CF1), W_(CF2), W_(BM), L_(F1),L_(F2) and L_(M) are as defined above.
 4. A laminate film as recited inclaim 3, and having a total thickness of 8-50 μm and wherein thethickness of each of the surface layers is 5-33% of the total thickness.5. A laminate film as recited in claim 1, and satisfying the followingconditions (d′) and (e′): (d′)0.04≦{(D_(CF1)−D_(AF1))×L_(F1)×W_(CF1)}+{(D_(CF2)−D_(AF2))×L_(F2)×W_(CF2)}≦0.12,(e′) 0.70≦(D_(AM)−D_(BM))×L_(M)×W_(BM)}≦1.20, wherein D_(CF1), D_(AF1),D_(CF2), D_(AF2), D_(AM), D_(BM), W_(CF1), W_(CF2), W_(BM), L_(F1),L_(F2) and L_(M) are as defined above.
 6. A laminate film as recited inclaim 5, and having a total thickness of 8-50 μm and wherein thethickness of each of the surface layers is 5-33% of the total thickness.